Results of postcolumn infusion experiments enable thescientist to evaluate the influence of different sampleextraction techniques on matrix effects, the appropriateanalytical column, where matrix effects occur and are absentduring a chromatographic run, the mechanistic aspect ofmatrix effects, and the influence of mobile additives onresponse. As an example, Fig. 2 shows a comparison of aninjection of (A) mobile phase, (B) a whole blood sampleprepared by protein precipitation, and (C) a whole bloodsample by solid phase extraction [33]. The analytesirolimus, an immunosuppressant drug (50 Ag/L), wasinfused (10 AL/min) postcolumn under the chromatographicconditions described in Taylor and Johnson [33]. Sirolimuswas monitored by selected reactant monitoring using thefollowing mass transition: m/z 931.6Y864.6. The retentiontime of sirolimus under these chromatographic conditionswould be approximately 6 min. For mobile phase injection(Fig. 2A), no change in signal was observed throughout thechromatogram. For the sample treated with acetonitrile toprecipitate proteins (Fig. 2B), the signal is suppressed forthe majority of the chromatogram. While the sampleprepared by solid phase extraction (Fig. 2C) showedminimal suppression in the solvent front but constant andmaximum signal at the retention time of the analyte.